wip: refactor offline download (#5331)

* base tool

* working: aria2

internal/offline_download/all_test.go

@@ -0,0 +1,13 @@
+package offline_download
+
+import "testing"
+
+func TestGetFiles(t *testing.T) {
+	files, err := GetFiles("..")
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, file := range files {
+		t.Log(file.Name, file.Size, file.Path)
+	}
+}